Distribution of indigenous trees

  • Image, Distribution of indigenous trees.

    The rates of establishment (recruitment) and death (mortality) of indigenous tree species vary across New Zealand. Changes in the state of the environment (such as from browsing pests, large-scale weather events, or climate change) may change the rates of recruitment and mortality of particular tree species. This in turn may alter forest processes. Repeated surveys of the distribution of recruitment and mortality rates can alert us to impacts on our indigenous forests.

    We classified Distribution of indigenous trees as a national indicator.

    Key findings

     Trend not assessed

    Eight of the 13 indigenous tree species (62 percent) investigated in two surveys, which took place between 2002 and 2014, showed no clear pattern in the distribution of their recruitment and mortality rates across New Zealand.

    • At a national scale, the rates of recruitment and mortality of most of New Zealand’s common forest tree species are in balance, suggesting that their populations are stable.
    • Halls tōtara and tree fern (whekī) both showed clear patterns of recruitment. Between the two surveys, higher-than-average numbers of trees became established towards the north of where these species are commonly expected to be found (ie their range).
    • Kāmahi had a very clear mortality pattern. Between the two surveys, much higher-than-average numbers of the tree died towards the north of its range.
    • Silver beech and māhoe showed clear patterns of both recruitment and mortality (population turnover). Between the two surveys, there was a high rate of recruitment and mortality towards the north of the ranges of both species.
    • Many factors influence recruitment and mortality rates, for example:
      • latitude – more sunlight can result in high rates of population turnover
      • forest disturbance – soon after a big disturbance (eg tropical Cyclone Ita), recruitment rates tend to be high

      • soil fertility – fertile soils can support more dynamic forests than nutrient-poor soils can.

    Distribution of indigenous tree species, 2002–14 – interactive map

    Definition and methodology

    Distribution of indigenous trees provides information on the recruitment and mortality rates of 13 tree species that occurred in at least 20 percent of survey plots in forests across New Zealand. These common species were Halls tōtara (Podocarpus cunninghamii), tree fern (Dicksonia squarrosa), kāmahi (Weinmannia racemosa), māhoe (Melicytus ramiflorus), silver beech (Lophozonia menziesii), red beech (Fuscospora fusca), black beech (Fuscospora solandri), putaputawētā (Carpodetus serratus), mountain horopito (Pseudowintera colorata), hūpiro (Coprosma foetidissima), kāpuka (Griselinia littoralis), lancewood (Pseudopanax crassifolius), soft tree fern (Cyathea smithii).

    This information comes from 874 survey plots (20m x 20m) distributed across forests on both public conservation land and private land around New Zealand. Each of these plots was first surveyed between 2002 and 2007. All trees with trunk diameters greater than or equal to 2.5cm when measured at 1.35m height (called ‘diameter at breast height’ or DBH) were tagged and identified. The same plots were resurveyed between 2009 and 2014. Any trees present in the first survey but missing from the second were recorded as dead. Any trees that died between surveys were also recorded as dead. Any trees not recorded in the first survey but recorded in the second as having a trunk greater than or equal to 2.5cm DBH were recorded as newly established trees.

    Statistical analyses were carried out to assess whether there were spatial patterns in the data and whether these patterns were associated with major environmental gradients, such as latitude and/or modelled soil moisture.

    Data quality

    We classified Distribution of indigenous trees as a national indicator.

    Relevance

    relevance-direct This national indicator is a direct measure of the ‘Land species, taonga, and genetic diversity' topic.

    Accuracy

    accuracy-high The accuracy of the data source is of high quality.

    See Data quality information for more detail.

     

    Published 21 October 2015

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